Long-term trends in particulate sulfate at Grand Canyon National Park

Abstract

Visibility impairment, particularly at the iconic southwestern U.S. national parks, was an early and persistent concern that helped motivate parts of the Clean Air Act (CAA) and its 1977 and 1990 amendments. Grand Canyon National Park (GCNP), Arizona, in particular, was the site of many early air quality studies that found that before 1990 particulate sulfate was the largest contributor to light extinction at GCNP. Since a peak in the 1960s, sulfate concentrations at GCNP have declined dramatically as sulfur dioxide (SO2) emissions in the southwestern United States and northern Mexico have been reduced, often due to successful regulations. The relative importance of upwind source regions has also changed. During the 1960s and 1970s the predominant SO2 emitters were copper smelters in Arizona and surrounding states. As smelter emissions declined, in the 1980s and 1990s electric generating units (EGUs) became the largest upwind SO2 sources. More recently, EGU emissions have also been reduced, especially since about 2000. Three back trajectory methods are used to examine the sulfate concentrations at GCNP as a function of upwind source regions. 1) Residence time analysis shows that Arizona, southern California, southern Utah, and northern Mexico are most often upwind of GCNP. There were highly significant reductions in mean sulfate concentrations at GCNP when air arrived after transport through most source regions. 2) Concentration trends by source region show that the maximum rates of decline for 1980–2018 were about −2%/year when there was transport through Colorado and the Navajo Generating Station (NGS) source regions. 3) The Trajectory Mass Balance (TrMB) model was used to apportion sulfate seasonally for 3-year rolling averages during 1981–2017. As expected, relative source attributions have evolved through time. Largest mean contributors in summer are northern Mexico (25–70%), Baja (0–25%), Arizona smelters (0–35%), the Mohave Generating Station (<10–18%) and Southern California (1–15%). During winter the Navajo Generating Station (10–25%) and Cholla (3–12%) often contributed the largest fractions. Mean concentrations are much lower, and there are larger fractional declines in contributions from many source areas during winter as compared to summer. These analyses document the successful outcomes that can result from effective regulations.